This application is based on Patent Application No. 2000-216694 filed Jul. 17, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing apparatus and method, and in particular, to a printing apparatus and method suitable for use in ink jet printing systems.
2. Description of the Related Art
Serial-scanning printing apparatuses that perform printing operations while scanning a print head on printing medium have been adapted to various image printing applications. In particular, ink jet printing apparatuses have been spreading rapidly due to their recently increased resolution and recently improved color printing function, which lead to an improved image grade. The serial-scanning printing apparatuses sequentially print images on printing medium by repeating a printing operation of printing an image on a printing medium while moving a print head in a main-scanning direction and a transporting operation of transporting the printing medium in a sub-scanning direction.
The serial-scanning ink jet printing apparatuses use, as a print head, a multi-nozzle head having a plurality of ejection ports integrated and arranged therein and constituting nozzles capable of ejecting ink droplets. Images can be printed with a higher resolution by increasing the integration density of the ejection ports and reducing the amount of ink ejected per dot. Further, for high-quality image printing equivalent to silver salt photographing, many techniques have been developed; for example, in addition to four basic color inks (cyan, magenta, yellow, and black), lighter-color inks of lower densities are provided, so that a total of six color inks (cyan, magenta, yellow, black, light cyan, and light magenta) are used for printing. Moreover, to avoid a decrease in printing speed associated with the increased image quality, techniques have been employed which increase the number of printing elements deployed in an arrangement including the ejection ports, increase the driving frequency for the print head, or enable bi-directional printing in which a printing operation is performed when the print head is scanned in either direction. As a result, the high throughput has been improved.
For these serial-scanning ink jet printing apparatuses, various proposals have been made for the construction of the print head and the printing method in order to deal with higher-resolution image printing.
FIGS. 11 to 13 are illustrations of an example of a construction of an ink jet print head H as a multi-nozzle head which includes nozzles arranged at a higher density to achieve high-density image printing. For the multi-nozzle head, a one-row nozzle arrangement in which ejection ports are arranged in a row has a limited nozzle arrangement density due to a manufacturing method used. Thus, in the print head H shown in FIG. 11, a plurality of ejection ports P capable of ejecting ink are formed so as to constitute two rows (hereafter also referred to as xe2x80x9cnozzle rowsxe2x80x9d) L1 and L2. The nozzle rows L1 and L2 extend in the sub-scanning direction shown by an arrow B in which printing medium is transported, and the ejection ports P are arranged at a predetermined pitch Py in each of the nozzle rows L1 and L2. An arrow X indicates a main-scanning direction in which the print head H reciprocates. The ejection ports P in the nozzle row L1 are offset from the ejection ports P in the nozzle row L2 by half a pitch (Py/2) in the sub-scanning direction. This serves to achieve a resolution twice as high as that achieved by single nozzle row. If, for example, six color inks are used to print image, six print heads H for ejecting the corresponding inks are provided in the sub-scanning direction. And, for each of the print heads H, timings for ejecting the ink from the ejection ports P in the nozzle rows L1 and L2 are adjusted. When image is thus printed using the one color ink from the two nozzle rows L1 and L2, color image of a double resolution can be printed compared to the one color ink from the one nozzle row.
On the other hand, the printing resolution of the printing apparatus does not always equal the resolution of image data input to the printing apparatus from a host apparatus (the latter resolution is hereafter referred to as an xe2x80x9cinput resolutionxe2x80x9d). The recent printing apparatuses can perform printing operations corresponding to a plurality of input resolutions. If, for example, the printing apparatus has a printing resolution of 1,200 dpi (dot/inch), the processing time and data transfer time of the host apparatus can be reduced when it processes image data at a resolution of 300 ppi (pixel/inch) and transfers it to the printing apparatus. If the host apparatus processes the image data at a resolution of 1,200 ppi correspondingly to the printing resolution of the printing apparatus, and transfers the data to the printing apparatus, then the host apparatus is overloaded. If the host apparatus processes the image data at a resolution of 300 ppi, one-fourth of 1,200 ppi, and transfers it to the printing apparatus, then the printing apparatus can print the image data in a printing area of 4xc3x974 pixels while applying gradation thereto.
Such a printing method is described, for example, in Japanese Patent Application Laid-open No. 9-46522 (1997). FIG. 15 is an illustration of an example of that printing method. In the example in FIG. 15, the printing apparatus prints an image at a resolution of 600 dpi on the basis of image data of 300 ppi resolution transferred from the host apparatus. If the image data of 300 ppi input resolution is printed with the resolution unchanged, the printing resolution is 300 dpi. Thus, the example in FIG. 15 also corresponds to the case in which the printing apparatus prints an image at a resolution of 600 dpi on the basis of image data of 300 ppi resolution transferred from the host apparatus.
The printing apparatus uses an arrangement pattern (hereafter referred to as a xe2x80x9cdot arrangement patternxe2x80x9d) of dots D in a printing area of 2xc3x972 pixels to achieve printing with five-value gradation from xe2x80x9clevel 0xe2x80x9d to xe2x80x9clevel 4xe2x80x9d, as denoted by reference signs (a) to (e) in FIG. 15. A plurality of dot arrangement patterns are used for the xe2x80x9clevel 1xe2x80x9d, xe2x80x9clevel 2xe2x80x9d, and xe2x80x9clevel 3xe2x80x9d. Japanese Patent Application Laid-open No. 9-46522 (1997) describes the sequential and random use of such a plurality of dot arrangement patterns. As denoted by reference signs (b), (c), and (d) in FIG. 15, the arrangements of the dots D for expressing the xe2x80x9clevel 1xe2x80x9d, xe2x80x9clevel 2xe2x80x9d, and xe2x80x9clevel 3xe2x80x9d gradations are not fixed. Thus, preventing the movement of the inks on a printing medium, for example, pseudo contours resulting from a pseudo half toning process or what is called xe2x80x9csweep-up phenomenonxe2x80x9d occurring at edge portions of the image. Further, the frequency of the use of the print head nozzles can be leveled out.
Such a printing method is particularly effective on printing apparatuses of a high printing resolution. For high-quality image printing equivalent to photographing, the input resolution need not exceed a visually perceived range. As long as an input resolution of about 600 dpi is obtained, increasing the gradation of pixels is more effective than further increasing the input resolution. Moreover, smooth and less granular images can be printed by using the above described six color inks including the light color inks to improve the gradation.
Further, as the printing density increases with the resolution, the throughput may decrease. To prevent this, what is called a column thinning printing system or the like has been proposed in addition to the above described increase in the number of nozzles, increase in the driving frequency for the print heads, and proposal for bi-directional printing.
Next, the column thinning printing system will be described. Typically, in the serial-scanning ink jet printing apparatus, the speed at which a carriage with print heads mounted thereon moves in the main-scanning direction is determined by the frequency with which the inks are ejected depending on the driving frequency for the print heads and by a basic resolution. With the column thinning printing method, a printing operation is performed while moving the carriage at a speed higher than such a predetermined one. That is, the carriage is moved in the main-scanning direction, while a thinned image is printed on a printing medium by the print heads. Subsequently, the printing medium is transported a predetermined distance in the sub-scanning direction, and the carriage is then moved in the main-scanning direction, while a portion of the image which has not been printed yet is printed by the print heads. That is, the image to be printed is divided into a plurality of complementary portions, which are then printed during a plurality of scanning operations preformed by the print heads.
For example, in a 2-pass bi-directional column thinning printing method, the movement speed of the carriage is set twice the typical predetermined value, whereas the driving frequency for the print heads are set at a typical value. Then, as shown in FIG. 16, if it is assumed that a lattice corresponding to the basic printing resolution is set on the printing medium and if pixels to be printed at intersections of the lattice (hereafter referred to as xe2x80x9cbasic lattice pointsxe2x80x9d) are expressed as black and white circle portions, then the black circle portions are printed during the first scanning operation of the print head (first pass). Subsequently, the printing medium is transported in the sub-scanning direction a distance equal to half the length of the nozzle row of the print head, and the white circle portions in FIG. 16 are printed during the second scanning operation of the print head (second pass). In this example, the black circle portions in FIG. 16 are printed during forward scanning in which the print head move in the forward direction (forward printing), whereas the white circle portions in the same figure are printed during backward scanning in which the print head move in the backward direction (backward printing). Further, in this example, the movement speed of the carriage is doubled with the printing resolution remaining equal to the basic printing resolution. The printing resolution in the main-scanning direction, however, can be increased by reducing, for each main-scanning operation of the print head (for each pass), the intervals between the printing pixels in the main-scanning direction below the distance between the basic lattice points in FIG. 16 with the movement speed of the carriage remaining unchanged. Alternatively, both of these methods can be used to increase both the movement speeds of the carriage and the printing resolution.
If, however, the six color inks including the dark and light inks are used in order to improve the quality of printed image, the light inks serve to eliminate the granularity in low density areas of the image, whereas the granularity remains in gradation changing portions between an area printed by a light color ink and an area printed by a dark ink. This is because in a gradation area expressed by a light ink, a dark ink applied to that area is noticeable. Additionally, a sufficient image density may not be obtained even through the inks have been applied to all the printing lattice points.
Further, as described previously, with the print head H shown in FIG. 11, even and odd rasters alternatively arranged in the sub-scanning direction shown by an arrow Y are printed by the different nozzle rows L1 and L2. Thus, if points on the printing medium where ink droplets impact are slightly offset between the nozzle rows L1 and L2, the image grade may lower. The causes of the offset of the ink droplet impact points include errors in the formation of the ejection ports P during the manufacturing of the print head H and the thermal deformation of a head face of the print head H with the ejection ports formed therein. That is, when the head face is deformed due to the ink or the ambient temperature, each of the directions in which ink droplets Ixe2x80x2 are ejected from the ejection ports P in the nozzle rows L1 and L2 changes as shown by the alternate long and two short dashes line in FIG. 13. In this figure, the ejection directions of the ink droplets Ixe2x80x2 change so as to form an inverse V shape, that is, they shift rightward and leftward in the figure relative to the normal directions shown by the solid lines in the figure. On the other hand, contrary to FIG. 13, the ejection directions of the ink droplets Ixe2x80x2 may change so as to form a V shape relative to the normal ones shown by the solid lines in the figure.
In the print head H in FIG. 13, h denotes a heater (electrothermal converter) that generates thermal energy used as ejection energy for the ink droplets Ixe2x80x2 in response to a driving signal. The thermal energy from the heater h causes film boiling in ink I in the nozzle, and resultant bubbling energy causes the ink droplets Ixe2x80x2 to be ejected from the ejection ports P. Further, in the print head H, the ejection directions of the ink droplet Ixe2x80x2 may shift along the passage direction of the ink I due to an increase in the ejection force for the ink droplets Ixe2x80x2 which increase is associated with an increase in temperature, thereby changing the ejection direction as shown by the alternate long and two short dashes line in FIG. 13.
The image printing grade is adversely affected by the offset of the impact positions of the ink droplets which results from the above phenomena, that is, the offset of the impact positions of the ink droplets occurring between the odd raster for which dots are formed by one of the nozzle rows L1 or L2 and the even raster for which dots are formed by the other nozzle row L2 or L1, even if the degree of the offset is small. In particular, if a high-resolution image is printed on the basis of image data obtained by a binarization method such as an error diffusion method, the printed image is significantly degraded.
Further, many proposals have been made for methods of correcting, for each ink color, the offset of the impact positions of ink droplets ejected from the print head or correcting, in the case of the bi-directional printing, the offset of the impact points of the same color ink between the forward scanning and the backward scanning. No effective adjustment methods, however, have been proposed for the correction of the offset of the impact points of the same color ink between adjacent rasters which offset occurs if the print heads H as shown in FIGS. 11 to 13 are used, though the tolerable range of the offset is small and it severely degrades the printed image.
Moreover, the offset of the ejection directions of the ink droplets Ixe2x80x2 from the ejection ports P in the nozzle rows L1 and L2 as shown by the alternate long and two short dashes line in FIG. 13 is aggravated by individual differences among the print heads H occurring during manufacturing, as well as the compositions of the ink, histories such as the ejection frequency of the ink droplets Ixe2x80x2, or environments present during the printing operation. For example, in continuous printing operations, an increase in the temperature of the print head H may cause a decrease in the viscosity of the inks, an increase in ejection force, a change in ejection angle, and an increase in ejection speed, leading to the offset of the ejection directions of the ink droplets Ixe2x80x2. The offset of the ejection directions varies with an increase in the temperature of the print head H during the printing operation, and returns to its original state after the printing operation has been completed, when the temperature of the print head H lowers. Thus, such changes in the situation cannot be dealt with even if the printing apparatus is provided with a mechanism for allowing a user to adjust the ejection directions.
Further, the above described technique described in Japanese Patent Application Laid-open No. 9-46522 (1997) is not intended to eliminate the offset of the impact positions of the ink droplets between the rasters and thus fails to solve this problem. Additionally, as described in this publication, the above described effect is expected to be obtained if the dot arrangement pattern is varied randomly. However, this requires a circuit for randomly generating a plurality of arrangement patterns, and this circuit must be relatively complicated. Moreover, if a plurality of arrangement patters are thus randomly generated, since a memory that supplies these plurality of arrangement patterns has a limited capacity, variations in arrangement pattern become significantly periodic, and this periodicity is expected to be noticeable on the printed image.
Further, if the column thinning printing method is employed in order to achieve printing while avoiding reducing the throughput as described previously, the impact positions of the ink droplets may be offset between the rasters due to the offset of the dot arrangements between complementary passes. Alternatively, with the bi-directional printing system, when a color image is printed using the color inks, the ink ejection order may vary with the scanning direction of the print heads. Consequently, in particular in high-density printed areas of the printing medium, coloring may vary with the order that the color ink dots are placed on one another, resulting in uneven colors, which degrade the image quality.
The present invention is provided in view of these circumstances, and it is an object thereof to provide a printing apparatus and method that can prevent the image grade from lowering by reducing the adverse effects of the variation of the positions of dots formed by a plurality of printing elements, in order to restrain the offset of dot forming positions between rasters.
In a first aspect of the present invention, there is provided a printing apparatus for printing with using a print head provided with a plurality of printing elements deployed in a plurality of rows and which can form dots on a printing medium, to print dots on N adjacent rasters and dots on M adjacent columns under different conditions by causing the print heads to perform a plurality of (P) main-scanning operations in a main-scanning direction and transporting the printing medium at least once in a sub-scanning direction, the apparatus comprising:
control means for using dot arrangement patterns corresponding to a level of quantized image data to form dots corresponding to the level of the image data on the printed medium, the control means being capable of periodically changing the plurality of dot arrangement patterns used for the same level of the image data, wherein
the plurality of dot arrangement patterns used for the same level of the image data are such that within each period when the patterns are periodically used, the number of dots formed in each of the N rasters are equalized, whereas the number of dots formed in each of the M columns are equalized, and
the P, N, and M are each an integral equal to or larger than 2.
In a second aspect of the present invention, there is provided a printing apparatus for printing with using a print head provided with a plurality of printing elements deployed in a plurality of rows and which can form dots on a printing medium, to print dots on N adjacent rasters and dots on M adjacent columns under different conditions by causing the print heads to perform a plurality of (P) main-scanning operations in a main-scanning direction and transporting the printing medium at least once in a sub-scanning direction, the apparatus comprising:
control means for using dot arrangement patterns corresponding to a level of quantized image data to form dots corresponding to the level of the image data on the printed medium, the control means can set one of the dot arrangement pattern which is used for the same level of the image data,
the one of the dot arrangement pattern which is used for the same level of the image data is such that an area of a surface on which dots are formed in each combination of the 1 to N rasters and the 1 to M columns using the dot arrangement pattern occupies 90% or more of a printing surface of the printing medium which corresponds to a printing range for the dot arrangement pattern, and
the P, N, and M are each an integral equal to or larger than 2.
In a third aspect of the present invention, there is provided a printing method for printing with using a print head provided with a plurality of printing elements deployed in a plurality of rows and which can form dots on a printing medium, to print dots on N adjacent rasters and dots on M adjacent columns under different conditions by causing the print heads to perform a plurality of (P) main-scanning operations in a main-scanning direction and transporting the printing medium at least once in a sub-scanning direction, comprising the steps of:
using dot arrangement patterns corresponding to a level of quantized image data to form dots corresponding to the level of the image data on the printing medium; and
periodically changing the plurality of dot arrangement patterns used for the same level of the image data, wherein
the plurality of dot arrangement patterns used for the same level of the image data are such that within each period when the patterns are periodically used, the number of dots formed in each of the N rasters are equalized, whereas the number of dots formed in each of the M columns are equalized, and
the P, N, and M are each an integral equal to or larger than 2.
In a fourth aspect of the present invention, there is provided a printing method for printing with using a print head provided with a plurality of printing elements deployed in a plurality of rows and which can form dots on a printing medium, to print dots on N adjacent rasters and dots on M adjacent columns under different conditions by causing the print heads to perform a plurality of (P) main-scanning operations in a main-scanning direction and transporting the printing medium at least once in a sub-scanning direction, comprising the steps of:
using dot arrangement patterns corresponding to a level of quantized image data to form dots corresponding to the level of the image data on the printed medium; and
setting one of the dot arrangement patterns which is used for the same level of the image data, wherein
the one of the dot arrangement pattern which is used for the same level of the image data is such that an area of a surface on which dots are formed in each combination of the 1 to N rasters and the 1 to M columns using the dot arrangement pattern occupies 90% or more of a printing surface of the printing medium which corresponds to a printing range for the dot arrangement pattern, and
the P, N, and M are each an integral equal to or larger than 2.
The present invention adopts printing style using a print head provided with a plurality of printing elements positioned in a plurality of rows to print dots on N adjacent rasters and dots on M adjacent columns under different conditions by causing the print head perform a plurality of (P) main-scanning operations in a main-scanning direction and transporting a printing medium at least once in a sub-scanning direction, wherein a plurality of dot arrangement patterns used for the same level of image data are periodically changed and are such that within each period when the patterns are repeatedly used, the number of dots formed in each of the N rasters are equated, and the number of dots formed in each of the M columns are equated. Thus, the adverse effects of the dispersion of dot forming positions among a plurality of printing elements are reduced to restrain the offset of the dot forming positions between the rasters, thereby preventing the image grade from lowering.
The present invention also adopts printing style using a print head provided with a plurality of printing elements positioned in a plurality of rows to print dots on N adjacent rasters and dots on M adjacent columns under different conditions by causing the print head perform a plurality of (P) main-scanning operations in a main-scanning direction and transporting a printing medium at least once in a sub-scanning direction, wherein one dot arrangement pattern is set which is used for the same level of image data and is such that a surface on which dots are formed in each combination of one of the 1 to N rasters and one of the 1 to M columns using that dot arrangement pattern occupies 90% or more of a printed surface of a printing medium which corresponds to a printing range for the dot arrangement pattern. Thus, the adverse effects of the dispersion of dot forming positions among a plurality of printing elements are reduced to restrain the offset of the dot forming positions between the rasters, thereby preventing the image grade from lowering.